Multiple channel driver for fiber optic coupled sensors and switches

ABSTRACT

A multiple channel driver for fiber optic devices includes a source and an associated detector for each channel having processing circuitry and the power for the sources common to all channels. The system includes a master controller which has a precise time base to sequentially access each channel. The master controller is used to coordinate the multiple channels in a selected sequential manner. Synchronization of the channels avoids crosstalk since only one channel is on at a time. Power requirements are reduced since only one source is on at a time. The controller measures the background noise level, and subtracts it from the received signal. If the background noise level is too high, or the received signal too low, the master controller provides a warning. The controller also considers the signal history as an aid in locking onto signals and providing cleaner switching. For example, if a signal has been received at an initial threshold level on a channel, then the next time the channel is monitored the signal will be accepted at a lower threshold level.

This invention relates to fiber optic systems and, in particular, to amultiple channel driver for fiber optic sensor and switch circuits.

BACKGROUND OF THE INVENTION

The use of fiber optics in control systems offers many operationaladvantages over similar electrical devices. Fiber optic circuits areelectrically passive, operating only with light. As such they offerunprecedented safety, immunity to noise, and ease of installation, allof which translate into lower costs and increased reliability. Physicaldesign requirements are greatly simplified since fiber optic cables andcomponents can be located adjacent to existing electrical installationswithout regard to interference from the electrical system or interactionbetween the two systems. Accordingly, fiber optic coupled sensors andswitches are used in many industrial, commercial and residentialapplications.

Generally, fiber optic circuits require a fiber optic driver. A typicalfiber optic driver includes, as a minimum, a source of light, a detectorof light and electronic circuitry to process the light signal beingdetected and to determine the state of the fiber coupled device beingdriven. Drivers may provide an output interface and additional functionssuch as background compensation or noise filtering.

It is possible to make a multiple channel driver by simply packagingmultiple single channel drivers together. However, this results inseveral undesirable consequences. Two primary problems are channelcrosstalk and peak power demands.

Channel crosstalk occurs when multiple independent drivers operate withmodulation frequencies or synchronization times that are sufficientlyclose that their detection circuits cannot distinguish them. As aresult, inaccurate switching can occur.

The optical sources typically used in fiber optic coupled driversconsume most of the electrical power. When independent devices are used,many of the fiber optic sources may be operated at the same time.Accordingly, the power supply must be designed to meet the currentdemands of multiple sources activated simultaneously. It is desirable toprovide the benefits of a multiple channel driver while avoiding thelimitations of the prior art systems.

SUMMARY AND ADVANTAGES OF THE INVENTION

The multiple channel driver of the present invention includes a sourceand an associated detector for each channel having processing circuitryand the power for the sources common to all channels. The systemincludes a master controller which has a precise time base tosequentially access each channel. The master controller is used tocoordinate the multiple channels. Since the master controller has aprecise time base, it switches each channel on and off in a selectedsequential manner. This synchronizes the source and detector for eachchannel and avoids crosstalk since only one channel is on at a time.Power requirements are reduced since only one source is on at a time.Significant cost savings are realized since drive circuitry and signalprocessing circuitry are shared by multiple channels.

The master controller further performs additional processing of thesignals to provide noise immunity, background compensation, and cleanerswitching. The controller measures the background noise level, andsubtracts it from the received signal. If the background noise level istoo high, or the received signal too low, the master controller providesa warning.

The controller also considers the signal history as an aid in lockingonto signals and providing cleaner switching. For example, if a signalhas been received at an initial threshold level on a channel, then thenext time the channel is monitored the signal will, be accepted at alower threshold level. Conversely, if a signal has not been previouslyreceived on the channel, the initial threshold level must be attained inorder for the signal to be accepted. This feature of "locking onto" thedetected signal ensures clean positive switching even in cases when thesignal-to-noise ratio is low.

Other objects and advantages of the present invention will be apparentto those of ordinary skill in the art from the following description ofa presently preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a multiple channel fiber optic multiplexerand demultiplexer system.

FIG. 2 is a detailed block diagram of the multiple channel fiber opticmultiplexer and demultiplexer system of FIG. 1.

FIG. 3 is a block diagram of a multiple channel optical circuit driver.

FIG. 4 is a flowchart of the process utilized by the multiple channeloptical circuit driver to detect channel states.

FIG. 5 is a schematic diagram of the multiple channel optical circuitdriver.

FIG. 6 is a schematic diagram of the optical signal processor anddetectors.

FIG. 7 is a schematic diagram of the optical multiplexer.

FIG. 8 is a flowchart of the process utilized by the optical multiplexerto access the information stored in the latches.

FIG. 9 is a graphical representation of the formatted signal transmittedby the optical multiplexer.

FIG. 10 is a schematic diagram of the optical demultiplexer.

FIG. 11 is a flowchart of the process utilized by the opticaldemultiplexer to receive and decode information transmitted by themultiplexer.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIG. 1, a multiple channel fiber optic multiplexer anddemultiplexer system 10 is shown. The fiber optic system 10 comprises aplurality of optical devices 12.1, 12.2 . . . 12.i, such as opticalswitches, a plurality of optical fiber pairs 14.1, 14.2 . . . 14.i, aplurality of I/O interfaces 17.1, 17.2 . . . 17.i, an opticalmultiplexer 16, a single optical fiber 18, an optical demultiplexer 20,a second plurality of I/O interfaces 21.1, 21.2 . . . 21.i, and aplurality of electrical outputs 22.1, 22.2 . . . 22.i. Each opticaldevice 12.1, 12.2 . . . 12.i and associated optical fiber pair 14.1,14.2 . . . 14.i comprises a single channel optical circuit 15.1, 15.2 .. . 15.i. The fiber optic system 10 is designed to handle a plurality ofchannels. In the preferred embodiment, the system multiplexes anddemultiplexes 64 channels and accordingly, up to 64 separate singlechannel optic devices can be operated via the system 10.

Each optical device 12.1, 12.2 . . . 12.i is coupled to the opticalmultiplexer 16 via an optical fiber pair 14.1, 14.2 . . . 14.i. Variousoptical devices can be driven, for example, switches, thermostats, orposition sensors. For convenience, the system 10 is described coupled tooptical switch circuits. The lengths of the optical fiber pairs 14.1,14.2 . . . 14.i may vary depending upon the actual physical location ofthe switch 12.1, 12.2 . . . 12.i in relation to the multiplexer 16. Thesingle bit data signal relating to the state (on or off) of an opticalswitch 12.1., 12.2 . . . 12.i is transmitted over the optical fiber pair14.1, 14.2 . . . 14.i, through an I/O interface 17.1, 17.2 . . . 17.i tothe optical multiplexer 16. The I/O interface 17.1, 17.2 . . . 17.itypically comprises a pair of fiber optic connectors or a singleconnector capable of connecting a pair of optic fibers.

The optical multiplexer 16 multiplexes the data signals into apredetermined format, and transmits the data via a multiplexed opticalsignal to an optical demultiplexer 20 over a single optical fiber 18.The optical demultiplexer 20 receives the formatted data signal,demultiplexes the signal and sends the data to a plurality of I/Ointerfaces 21.1, 21.2 . . . 21.i, which are coupled to a plurality ofelectrical outputs 22.1, 22.2 . . . 22.i. The type of output interface21.1, 21.2 . . . 21.i depends on the type of electrical output 22.1,22.2 . . . 22.i to be coupled with. For example, the electrical outputcould be a graphical display or the input to a logic system.

A more detailed block diagram of the fiber optic system 10 is shown inFIG. 2. The optical multiplexer 16 comprises a multiple channel opticalcircuit driver 24, an electrical multiplexer 26, and an optical source28. The multiple channel optical circuit driver 24 detects and convertsthe optical data signals received on each channel 15.1, 15.2 . . . 15.ito electrical data signals.

Although the fiber optic system 10 is capable of receiving a pluralityof single channel optical inputs, the preferred embodiment utilizes amultiple channel optical circuit driver 24. A more detailed blockdiagram of the multiple channel optical circuit driver 24 is shown inFIG. 3. The preferred embodiment is capable of driving up to 64channels, although the number of channels may vary according to therequirements of the specific application. The optical driver 24comprises an optical source 154.1, 154.2 . . . 154.i, an opticaldetector 156.1, 156.2 . . . 156.i, and an input/output (I/O) interface17.1, 17.2 . . . 17.i, for each channel 15.1, 15.2 . . . 15.i which arecollectively coupled to a drive circuit 160, a signal processor 162, amaster controller 164, a data output interface 166, and a warninginterface 167. In the preferred embodiment, separate optical emittingsources 154.1, 154.2 . . . 154.i and separate detectors 156.1, 156.2 . .. 156.i are provided for each channel 15.1, 15.2 . . . 15.i. However,optical sources and optical detectors serving multiple channels could beemployed in an arrangement such that each channel is served by adifferent pairing of optical source and optical detector.

Each source 154.1, 154.2 . . . 154.i and associated detector 156.1,156.2 . . . 156.i are coupled to an optical switch 12.1, 12.2 . . . 12.ivia an I/O interface 17.1, 17.2 . . . 17.i and a fiber optic pair 14.1,14.2 . . . 14.i. The master controller 164 coordinates the operation ofthe drive circuit 160 and the signal processor 162. The mastercontroller 164, selects the channel 15.1, 15.2 . . . 15.i to be drivenand monitored. In response to the master controller 164, the drivecircuit 160 energizes a source 154.1, 154.2 . . . 154.i which emitslight, and the signal processor 162 activates the associated detector156.1, 156.2 . . . 156.i to detect the presence or absence of a returnof at least a portion of the emitted light.

An optical data signal is generated in the following manner. The lightemitted from the optical source 154.1, 154.2 . . . 154.i travels throughthe I/O interface 17.1, 17.2 . . . 17.i, and through the first fiber ofthe optical fiber pair 14.1, 14.2 . . . 14.i, to the optical switch12.1, 12.2 . . . 12.i. If the switch 12.1, 12.2 . . . 12.i is in theopen position, no light that was emitted from the source 154.1, 154.2 .. . 154.i will be returned to the detector 156.1, 156.2 . . . 156.i. Ifthe switch 12.1, 12.2 . . . 12.i is in the closed position, the lightwill be reflected by the switch 12.1, 12.2 . . . 12.i and at least aportion of the light will return through the second fiber of the opticalfiber pair 14.1, 14.2 . . . 14.i to the detector 156.1, 156.2 . . .156.i. The detection of an absence or presence of returned light at thedetector 156.1, 156.2 . . . 156.i comprises the optical data signalindicative of the state (ON or OFF) of the optical switch 12.1, 12.2 . .. 12.i. The absence or presence of light at the detector could becreated mechanically by the optical device, such as by the depression ofa push-button switch which interrupts the light path, or reflectively byan object on a conveyor belt which reflects the emitted light signal orbreaks the path of the light beam as it passes the sensor.

A detector 156.1, 156.2 . . . 156.i may erroneously detect ambientlight, or noise, as the presence of returned light emitted by the source154.1, 154.2 . . . 154.i. Therefore, the fiber optic system 10 includesmeans to subtract out the noise generated by ambient light sources.Also, a minimum signal threshold level is established in order for thedetected light to be considered a valid signal.

If the detector 156.1, 156.2 . . . 156.i detects a light signal at theminimum threshold level, the state (on or off) of the channel 15.1, 15.2. . . 15.i is considered to be ON. However, if the detected light signaldoes not achieve the minimum threshold level, the state of the channel15.1, 15.2 . . . 15.i is considered to be OFF. In order to ensure thatthe state of each channel 15.1, 15.2 . . . 15.i is properly detected,each detector 156.1, 156.2 . . . 156.i must be activated while itsassociated source 154.1, 154.2 . . . 154.i is energized. Once the stateof the channel 15.1, 15.2 . . . 15.i is obtained, it is stored in anoutput interface 166.

The process utilized by the master controller 164 is shown in detail inFIG. 4. After the channel latch count is initialized, the mastercontroller 164 selects the first channel 15.1. The detector 156.1associated with the selected channel 15.1 is activated for apredetermined duration to read the background noise level. If thebackground noise level is too high, the master controller 164 generatesa first warning signal. This warning signal is output to a warninginterface 167. The drive circuit 160 energizes a source 154.1, 154.2 . .. 154.i associated with the selected channel 15.1, 15.2 . . . 15.i, andthe signal processor 162 activates the associated detector 156.1, 156.2. . . 156.i. The optical data signal detected by the detector 156.1,156.2 . . . 156.i is a combination of the signal relating to the stateof the channel 15.1, 15.2 . . . 15.i and the background noise level. Themaster controller 164 subtracts out the background noise to obtain thesignal relating to the state of the channel 15.1, 15.2 . . . 15.i. Inthe preferred embodiment, background compensation is sufficient to allowstandard fiber optic coupled retroreflective sensors to operate in10,000 lux of ambient light.

The master controller 164 considers the history of the state of thechannel as an aid in locking onto signals and providing cleanerswitching. For example, if the strength of a signal detected on achannel 15.1, 15.2 . . . 15.i exceeds an initial predetermined ONthreshold level, the channel 15.1, 15.2 . . . 15.i is considered to beON. The next time the channel 15.1, 15.2 . . . 15.i is read, the mastercontroller 164 will consider the channel 15.1, 15.2 . . . 15.i to be ONif the signal strength is greater than a "low signal" threshold which isless than the initial predetermined ON threshold level. This process isshown in FIG. 4.

Preferably, the initial ON threshold level is set at 25 nW and the "lowsignal" ON threshold level is set at 15 nW. If a channel 15.1, 15.2 . .. 15.i was previously ON, and the signal strength is greater than 15 nW,the channel 15.1, 15.2 . . . 15.i will still be considered ON. If thereceived signal is less than 15 nW, the channel 15.1, 15.2 . . . 15.iwill be considered OFF. If, however, the channel 15.1, 15.2 . . . 15.iwas previously OFF, the received signal strength must reach the ONthreshold level of 25 nW to consider the channel 15.1, 15.2 . . . 15.iON. This "locking on" feature ensures clean positive switching even ifthe signal-to-noise ratio is low. This process is explained withreference to the multiple channel optical circuit driver 24, however,this can also be utilized with a single channel device.

Although a channel 15.1, 15.2 . . . 15.i is considered ON when a signalis received at 25 nW, the master controller 164 will generate a secondwarning signal for that channel 15.1, 15.2 . . . 15.i when the signalstrength drops below a warning threshold, preferably 75 nW. This warningsignal is output to the warning interface 167. When the mastercontroller 164 determines that a valid signal has been obtained, themaster controller 164 stores the data, increments the channel latchcount, and proceeds to read the state of the succeeding channel 15.2 . .. 15.i.

Once all channels 15.1, 15.2 . . . 15.i have been read by the mastercontroller 164, the state of each channel 15.1, 15.2 . . . 15.i isdisplayed on the LED bargraph 165 as shown in the schematic of FIG. 5and output to the output interface 166 (not shown). Warning signals arealso displayed on a separate set of LED bargraphs 167.

Operation of the multiple channel optical circuit driver 24 is explainedwith reference to FIGS. 5 and 6. The master controller 164 includes amicroprocessor U1 which sends address information to the signalprocessor 162 shown in FIG. 6. The signal processor 162 selects thedetector 156.1, 156.2 . . . 156.i associated with the selected channel15.1, 15.2 . . . 15.i and the background noise level for the selectedchannel 15.1, 15.2 . . . 15.i is detected, amplified and sent to themicroprocessor U1. The microprocessor U1 then transmits the addressinformation of the channel 15.1, 15.2 . . . 15.i to the drive circuit160 and the selected source 154.1, 154.2 . . . 154.i is energized toemit a beam of light. The detector 156.1, 156.2 . . . 156.i is againactivated to detect the reflected light signal from the selected channel15.1, 15.2 . . . 15.i. This information is transmitted to themicroprocessor U1 for processing in accordance with the process shown inFIG. 4.

Examples of the various electrical elements depicted in the schematic ofFIG. 5 are given in Table 3, below:

    ______________________________________                                        Item                Value                                                     ______________________________________                                        C1,C2               15 pF                                                     C3                  100 uF                                                    R1                  100Ω                                                R2,R5               330Ω                                                R3                  6.2Ω                                                R4                  16Ω                                                 R6                  10.0 KΩ                                             J1                  CONNECTOR                                                 D1,D2,D3,D4,D5,D6,D7,D8                                                                           LED                                                       P1,P2,P3,P4,P5,P6,P7,P8                                                                           POTENTIOMETER                                             U1                  PIC16C55                                                  U2,U3,U5,U6         74HC573                                                   U4                  ULN2801                                                   Y1                  CRYSTAL                                                   S1                  DIP SWITCH                                                D9,D10              LED BAR GRAPH                                             ______________________________________                                    

Examples of the various electrical elements depicted in the schematic ofFIG. 6 are given in Table 4, below:

    ______________________________________                                        Item                   Value                                                  ______________________________________                                        C4                     5 pF                                                   C5                     .01 uF                                                 R7,R8                  1.0 MΩ                                           R9                     20.0 KΩ                                          R10                    158 KΩ                                           R11                    10.0 KΩ                                          D11,D12,D13,D14,D15,D16,D17,D18                                                                      PHOTODIODE                                             J2                     CONNECTOR                                              J3                     CONNECTOR                                              U7                     TLC274BCN                                              162                    MAX158BCPI                                             ______________________________________                                    

The program in the microprocessor U1 has the ability to detect, process,and output the states of eight channels in less than 800 μx. Themultiple channel optical circuit driver 24 is coupled to an electricalmultiplexer 26 as shown in FIG. 2 which multiplexes the data signalsinto a predetermined format. The multiplexed data signals are convertedback to an optical data signal by the optical source 28, fortransmission over the single optical fiber 18.

The optical demultiplexer 20 comprises an optical detector 30 coupled toan electrical demultiplexer 32. The optical detector 30 detects andconverts the optical data signals into electrical signals. Theseelectrical data signals are then demultiplexed by the electricaldemultiplexer 32 and outputted to the output interfaces 21.1, 21.2 . . .21.i.

Significant savings in cost and complexity are realized by the use ofprogrammable controllers 50, 51 in the optical multiplexer 16 andoptical demultiplexer 20. Many of the functions performed by discretecomponents in prior art systems are performed by programmablecontrollers 50, 51 in the present invention. Accordingly, relatively fewcomponents are utilized as compared to previous designs.

A schematic diagram of the optical multiplexer 16 is shown in FIG. 7.The data associated with each channel 15.1, 15.2 . . . 15.i is stored ina predetermined position among a plurality of latches U8, U9, U10, Ull,U12, U13, U14, U15. The programmable controller 50 includes amicroprocessor U16 that sequentially accesses each latch U8, U9, U10,Ull, U12, U13, U14, U15 on the select line using the process displayedin FIG. 8. After the latch count is initialized, the microprocessor U16selects the first latch U8 and reads the data contained in the latch U8.Each latch U8, U9, U10, Ull, U12, U13, U14, U15 stores data pertainingto eight channels 15.1, 15.2 . . . 15.i. The data is multiplexed into apredetermined format by the microprocessor U16 and sent to the opticalsource 28 for transmission over the single optical fiber 18. The latchcount is then incremented by one and the microprocessor U16 accesses thenext latch U9 to read the data contained therein. This process iscontinually repeated during the operation of the optical system 10.

Referring to FIG. 7, the optical source 28 is an LED D19 which iscoupled to the microprocessor U16. A plurality of LEDs D20, D21, D22,D23, D24, D25, D26, D27 each corresponding to a respective latch US, U9,U10, U11, U12, U13, U14, U15 to indicate when the respective latch US,U9, U10, U11, U12, U13, U14, U15 is being accessed by the microprocessorU16.

Examples of the various electrical elements depicted in the schematic ofFIG. 7 are given in Table 3 below:

    ______________________________________                                        Reference                Part Value                                           ______________________________________                                        C7                       1 uF                                                 C8,C9                    15 pF                                                D19,D20,D21,D22,D23,D24,D25,D26,D27                                                                    LED                                                  Q1                       2N3904                                               R21,R22,R23              10 KΩ                                          R24                      1 KΩ                                           R25                      500Ω                                           R26                      1 MΩ                                           R27                      60Ω                                            R28                      3.9 KΩ                                         R29,R30,R31,R32,R33,R34,R35,R36                                                                        300Ω                                           S2                       SW DIP-3                                             U8,U9,U10,U11,U12,U13,U14,U15                                                                          74LS373                                              U16                      PIC16C55                                             Y2                       CRYSTAL                                              ______________________________________                                    

The format of the optical signal transmitted by the optical source 28 isshown in FIG. 9. The information consists of a start pulse, (indicatedby S), a latch address identifier, (indicated by A), and the actual datarelating to the states of eight channels, (indicated by D). Theinformation is sent twice in each word frame for error detection by thedemultiplexer 20.

In the preferred embodiment, the multiplexer 16 operates at a speedwhich is greater than the speed of the multiple channel optical circuitdriver 24. This ensures that all sensing data detected by the sequencingof the optical driver 24 through the channels 15.1, 15.2 . . . 15.i istransmitted by the multiplexer 16.

A schematic diagram of the optical demultiplexer 20 is shown in FIG. 10.The demultiplexer 20 comprises an optical detector 30 coupled to anelectrical demultiplexer 32. The optical detector 30 detects the opticalsignal transmitted from the optical multiplexer 16. The programmablecontroller 51 in the electrical demultiplexer 32 contains amicroprocessor U20 monitors the optical detector 30 for a start pulse.The microprocessor U20 decodes the information contained in the opticalsignal by utilizing the process shown in FIG. 11. After a start pulse isdetected by the optical detector 30, the microprocessor U20 reads thesignal received from the optical detector 30 at precisely defined times.The microprocessor U20 demultiplexes and decodes the signal to determinethe actual data (D), the address (A) of the output latch U21, U22, U23,U24, U25, U26, U27, U28 to which the data should be sent, and whether anerror has occurred. In order for data to be considered valid, themicroprocessor U20 must receive the data twice. If the data isdetermined to be valid, the microprocessor U20 sends the data to thespecific output latch U21, U22, U23, U24, U25, U26, U27, U28. If data isnot received twice, no information will be output to the specific latchU21, U22, U23, U24, U25, U26, U27, U28.

Examples of the various electrical elements depicted in the schematic ofFIG. 10 are given in Table 4, below:

    ______________________________________                                        Reference                Part Value                                           ______________________________________                                        C10                      1 uF                                                 C11,C12                  15 pF                                                C13                      .1 uF                                                D30,D31,D32,D33,D34,D35,D36,D37,D38                                                                    LED                                                  R40                      1 KΩ                                           R41,R43                  500Ω                                           R42                      1 MΩ                                           R44                      1 KΩ                                           R45                      330Ω                                           U20                      PIC16C55                                             U21,U22,U23,U24,U25,U26,U27,U28                                                                        75LS374                                              U29                      HFD3023                                              U30                      ULN2801                                              Y3                       CRYSTAL                                              ______________________________________                                    

A plurality of LEDs D30, D31, D32, D33, D34, D35, D36, D37 eachcorresponding to a respective latch U21, U22, U23, U24, U25, U26, U27,U28 to indicate when a data error for the respective latch U21, U22,U23, U24, U25, U26, U27, U28 has been detected by the microprocessorU20. The microprocessor U20, outputs a signal to a single LED D38 toindicate when a system failure has occurred.

The embodiment as described is able to multiplex, send and demultiplex64 individual states in 400 μS through a length of fiber exceeding 1 Km.

The fiber optic system 10 is able to utilize moderate speed and stillobtain a high throughput for the subject application. The embodiment asdescribed is capable of a one megabit per second baud rate. After errorchecking and system overhead, the system 10 is capable of a throughputof 160K channels per second. This is the equivalent of the system beingable to multiplex, transmit and demultiplex 64 individual states in 400μs through a length of fiber exceeding 1 km.

We claim:
 1. A multiple channel optical driver for driving opticalindustrial and process control sensor circuits, each sensor circuitremotely located at the end of an optical fiber pair comprising:aplurality of individual optical channels, each having an optical outputand an associated optical input, for connection with a plurality ofoptical sensor circuits via an optical fiber pair such that each sensorcircuit receives an optical signal from the optical output of theoptical channel to which it is connected and controls a return opticalsignal to the associated optical input; means for emitting opticalsignals from said optical outputs in a selected sequential manner; meansfor detecting return optical signals communicated to said optical inputsin a corresponding selected sequential manner such that the emission ofan optical signal from each optical output is activated to correspondwith the detection of sensor data for its said associated optical input;said detecting means determines the intensity of each return opticalsignal; each said optical channel being designated as active after saiddetecting means detects a return signal from that channel with anintensity above a predetermined threshold level and being designated asinactive when said detecting means detects a return signal with anintensity below said predetermined threshold; and said predeterminedthreshold is selected based upon channel status such that saidpredetermined threshold is set at a first level when the channel isinactive and said predetermined threshold is set at a second level,lower than said first level, when the channel is active.
 2. A multiplechannel optical driver according to claim 1 wherein each optical outputis associated with a light emitting source and power is supplied to theplurality of light emitting sources in a sequential manner to providesaid emitted optical signals.
 3. A multiple channel optical driveraccording to claim 2 wherein each optical input is associated with anoptical detector, said optical detectors are associated with a datacollection means which receives data from said detectors in a sequentialmanner which corresponds with the sequence of emitted optical signals.4. A multiple channel optical driver according to claim 3 wherein aseparate light emitting source is associated with each optical outputand a separate optical detector is associated with each optical input.5. A multiple channel optical driver according to claim 1 whereindetecting means includes means for detecting background light intensitylevels on each channel at each said input before the optical signal isemitted from said output and means for subtracting the background levelfrom the detected signal to accurately detect the return optical signal.6. A multiple channel optical driver according to claim 1 wherein saidfirst level is 25 nW and said second level is 15 nW.